Mountain Ash
In "Teen Wolf" mountain ash also known as Sorbus_americana is a form of ash created from a tree that prevents supernatural creatures from crossing or touching. It was first introduced in Season 1. In real world mythology, it is often referred to as Rowan and supposedly has the ability to scare of evil beings. The mountain ash has also been said to keep supernatural creatures from causing trouble. The locked ward in Eichen House is filled with supernatural creatures, that part of the building is said to be made of mountain ash, it keeps the inmates from escaping. Usage Mountain ash is a powerful substance in druidic magic. It can be used to create a barrier that keeps out the supernatural. Mountain ash ashes are commonly used to lay magic circles that, when imbued with a force of will, can trap Werewolf and potentially other supernatural creatures. Various buildings are built out of Mountain Ash wood which works the same as the ash. With great effort, Scott McCall was able to pass through Mountain Ash but apparently it nearly killed him. Also with time, the Oni are able to search for weaknesses in the ash to break it. Supernatural creatures are not able to handle Mountain Ash in any way. Only humans, Druids and Banshees,chimeras are able to handle Mountain Ash and are able to activate it. Mountain Ash is also capable of stopping a transformation into a werewolf by ingesting it. Mythical Properties Mountain Ash is mainly used by Druids. It blocks and exhibits control over werewolves and other supernatural creatures. Mountain Ash Effect Effects Its typically used by Druids to not only prevent creatures from getting past but according to Deaton modify their behavior as well. The ash seems to magically form a perfect circle or line when used, this has happened on multiple occasions, for example when Melissa McCall used it at her house to keep the Oni out her house, suggesting that the ash can be used by anyone with very little effort. Ms. Morrell created a perfect circle of mountain ash when protecting herself against the Alpha Pack. When used correctly, it can form a almost impenetrable barrier that no supernatural creature can cross, if the barrier is touched after being completed it will create a very bright blue force field. However there have been a a couple instances of a supernatural creature breaking the barrier thus allowing them access. In "Lunar Ellipse" at the old distillery, potential True Alpha Scott McCall was able to force his way through it, he used all his force to break thought the barrier, this is something that was thought to be impossible. Another instance of a supernatural entity getting past the ash is when the Oni used their swords to find a weak spot and physically force their way through the mountain ash barrier. Other than those two instances, mountain ash has never been broken by a supernatural creature. Examples Druids Ms. Morrell and Jennifer Blake throw down circles of Mountain Ash dust to create a personal shield. Peter Hale could not pass while a balustrade made of the wood was down, completing the circle of Mountain Ash built into the Vet Clinic in Formality. Isaac and Erica could not cross a circle of Mountain Ash powder in Raving. Capsules filled with Mountain Ash powder and ingested by Gerard cause him to violently reject a werewolf bite in Master Plan. Dr. Deaton says his animal clinic is lined with Mountain Ash "making it difficult for someone like Scott to cause me any trouble." Sometime after Lunar Ellipse, Deaton installed Mountain Ash baseboards throughout the McCall home. Spreading Mountain Ash along front door jam completes the circle and (mostly) protects those inside from supernatural creatures. Lorraine Martin's boat house is apparently made of Mountain Ash. Spreading ash along the opening to the dock completed the circuit in Time of Death. The supernatural lockup at Eichen House, the cells and a section of hallways are lined with Mountain Ash to prevent creatures from getting in or out. (Smoke & Mirrors, A Novel Approach) Exceptions With great effort, Scott McCall was able to cross a Mountain Ash barrier. Peter Hale, and later Deucalion, showed that supernatural being can still throw items through a Mountain Ash barrier. Several Oni were able to break through the barrier around the McCall home. Tracy was able to cross Mountain Ash in her Kanima form because the rules don't apply to the Dread Doctors creations. Deputy Parrish was able to burn through a Mountain Ash barrier as a Hellhound. Doing so burns the Mountain Ash and effectively destroys the barrier. :